:root {
    --line-border-empty: #e0e0e0;
    --line-border-fill: #3498db;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    /* 弹性布局:垂直居中 */
    display: flex;
    justify-content: center;
    align-items: center;

    height: 100vh;
    background-color: #f6f7fb;
    overflow: hidden;
}

.container {
    /* 文字居中对齐 */
    text-align: center;
}

/* 进度条容器 */
.progress-container {
    /* 弹性布局:沿主轴 两边贴边,剩下平分 */
    display: flex;
    justify-content: space-between;

    position: relative;
    margin-bottom: 30px;
    max-width: 100%;
    width: 350px;
}

/* 空白进度条 */
.progress-container:before {
    content: '';
    background-color: var(--line-border-empty);

    /* 垂直居中 */
    position: absolute;
    left: 0;
    top: 50%;
    transform: translateY(-50%);

    height: 4px;
    width: 100%;

    /* z坐标为负,让其被数字圆圈压住 */
    z-index: -1;
}

/* 进度条 */
.progress {
    /* 与空白进度条一致,垂直居中 */
    position: absolute;
    left: 0;
    top: 50%;
    transform: translateY(-50%);

    height: 4px;
    width: 0%;

    /* 填充色为 蓝 */
    background-color: var(--line-border-fill);

    transition: .4s ease;
    /* 负z值,使其被 圆圈数字压住 */
    z-index: -1;
}

.circle {
    /* 让圆圈中的数字居中 */
    display: flex;
    justify-content: center;
    align-items: center;

    height: 30px;
    width: 30px;

    border: 3px solid var(--line-border-empty);
    border-radius: 50%;

    color: #999;
    background-color: #fff;

    transition: .4s ease;
}

.circle.active {
    border-color: var(--line-border-fill);
}

.btn {
    margin: 5px;
    padding: 8px 30px;

    border: 0;
    border-radius: 6px;

    font-size: 14px;
    color: #fff;
    background-color: var(--line-border-fill);

    cursor: pointer;
}

/* 按下按钮,缩小字体 */
.btn:active {
    transform: scale(.98);
}

/* 按钮聚焦时,轮廓线为0 */
.btn:focus {
    outline: 0;
}

/* 按钮被禁时的样式 */
.btn:disabled {
    background-color: var(--line-border-empty);
    cursor: not-allowed;
}